Disk drive memory systems ("disk drives") have been used to store information for computers for many years. In disk drives, digital information is recorded on concentric memory tracts on magnetic disks. There are two basic kinds of disk drives: "floppy" disk drives and "hard" disk drives. In both kinds of disk drives, disks are rotatably mounted on a spindle. Read/write heads, generally located on pivoting arms, move radially over the surface of the disks to access different memory locations. There may be as many as 3000 or more memory tracks per radial inch of a disk. To ensure proper writing and reading of the information, a read/write head must be accurately aligned with an appropriate track on a disk. Floppy disk drives store information on interchangeable, flexible and magnetic disks. Hard disks store information on rigid non-interchangeable disks, commonly made of aluminum. Hard disks have a much higher storage density than floppy disks.
Hard disks are often located within the housing of a computer and may consist of multiple rigid metal disks stacked on top of each other within the drive. The disks are spun at high speeds by a motor to allow rapid writing and reading of information on the disk. Because of the high storage densities of hard disks, it is extremely important that distortions and misalignments between the disks and the motor spindle, and among the disks themselves, be minimized to allow accurate information exchange between the read/write head and the disk.
Until recently, hard drive systems have not been portable and were housed in large rooms having controlled environments or within non-portable computers housed within such rooms. Consequently, the weight and space occupied by such systems were of little concern. Recently, however, the advent of personal and portable computer systems has placed a premium on providing disk drives of reduced size and weight. The relative size of disk drives is commonly referred to as its "form factor", and is expressed in terms of width. Currently, form factors of 2.5 inches and even smaller are relatively commonplace. Generally, a small form factor of a particular disk drive motor limits the torque generating capability as the magnet must be of a small size.
In-hub motors are presently used in disk drives and are particularly popular in smaller form factor disk drives. In-hub motors require less space than other conventional motors, because the hub the disks ride on carries an integral component of the actual motor, typically a permanent magnet. To produce the required electromagnetic characteristics, present designs for in-hub motors have required the use of hubs made of steel or other ferromagnetic materials. Other motors have an aluminum hub with a ferromagnetic sleeve. Magnets are mounted to the hubs such that the hub or sleeve provide a flux path for the magnets. The use of a steel hub in contact with aluminum disks, however, may create localized frictional sticking points from discontinuous thermal swelling and contracting. Subsequent displacement of the disks, and therefore potential misalignment of embedded servo tracks on the disk, will detrimentally cause the read/write heads to track an out-of-round path. Aluminum hubs, on the other hand, thermally expand faster than the typically steel bearings and motor base the hub rotates on, resulting in varying or excessive stresses on the bearings.
U.S. Pat. No. 4,814,652 to Wright, discloses a disk drive motor with thermally matched parts. The disk drive includes an aluminum hub which is compatible with the aluminum disks, and will not create distortion or misalignment upon thermal expansion or contraction. The drawback to this design is that the magnet within the aluminum spindle hub requires a magnetic flux return path which cannot be provided by the aluminum. Thus, a steel flux return sleeve is inserted inside the aluminum spindle hub adjacent the magnets. The reduction in thickness of the spindle hub wall due to the addition of a steel magnetic flux return sleeve, renders the walls of the spindle hubs susceptible to bending when a downward load is placed on the disks stacked on top of the spindle hub. Any such bending will cause a misalignment in the disks as well, detrimentally affecting the storage and retrieval of data.
An important factor affecting the performance of disk drives is the entry of contaminants, such as dirt, dust or moisture, into the housing. However, past attempts to hermetically seal the inner space of the disk drive has resulted in problems such as leaking at high altitudes because of pressure differences, and condensation within the inner space. Presently, disk drives use ambient or breather filters to prevent the entry of contaminants into the interior of the housing. Desiccants, or chemical absorbents, are occasionally used in these filters to absorb moisture before it enters the housing. Another related problem has been contaminants generated within the housing. Such contaminants would result from particulate matter flaking off components within the housing.
Gaseous contaminants, such as bearing greases containing lithium, can be very harmful when deposited on the disk drive surfaces. Unfortunately, lithium-based bearing greases have been preferred for their lower viscosity and lower running friction. Conventional small form factor spindle motors having low torque are currently limited to using low viscosity greases. However, when deposited on the disk surface, lithium rapidly increases the static coefficient of friction. Other hydrocarbons used in bearing greases can also out-gas, drifting around the inside of the disk drive housing, eventually being deposited on the disk surface. Out-gassed hydrocarbons are generated when the temperature of the bearing grease is raised and might otherwise deposit in a thin film on the surface of the aluminum disks.
Contaminants on the disk surfaces increase the static coefficient of friction of the disk under the magnetic head which can create a condition known as "stiction" or when the head sticks momentarily prior to lifting off from a stationary position on the disk. Large amounts of stiction may actually prevent smaller form factor/small torque motors from turning. That is, there is a relatively low limit to the amount of electrical power available in smaller form factor motors to increase torque and overcome such stiction. Additionally, the size requirements and the increased difficulty of manufacturing thin magnets of highly magnetic material limit the strength of permanent magnets available for in-hub motors.
Furthermore, cogging, or a wasteful motion of the motor hub tangential to the axis of rotation, is a problem with high-energy-product magnets, sometimes utilized to increase the torque of motors. Cogging vibrations are caused by the sudden changes in magnetic attractions when the poles of the magnet rotate around the discrete structural features of the ferromagnetic stator. Low-inertia hubs, such as in small form factor drives, increase the relative impact of torque variations caused by strong magnets.
Problems with liquid adhesive contamination of bearing greases has proved a difficult problem as well. Prior means for adhering bearings to spindle shafts utilize only one bonding groove, prompting variable filling of the grooves with adhesive. Excessive adhesive overflows as the bearing is pushed onto the shaft, or within a motor base flange. This adhesive overflow in many cases would then contaminate the bearing grease. One solution is to reduce the amount of adhesive deposited in the single groove to avoid pushing any excess outside of the bearing. However, this results in a less than solid or maximum bond between the bearing and the respective spindle or motor base surface, to the point that poor bond integrity may affect the maintenance of a consistent preload on the bearings.
Presently, there is a need for a small form factor disk drive motor which overcomes these problems.